1. Field of The Invention
The present invention relates to the field of audio signal processing technologies. More particularly, the present invention relates to the field of audio compression technologies.
2. Description of The Prior Art
Audio signals are complex waveforms which often contain transients of brief duration and relatively high amplitude. These transients are natural to sound, and cannot easily be avoided. Taking transients into account, the dynamic range of an audio signal can be wider than is practical for electronic recording, transmission, and reproduction. To avoid overload, noise, and distortion problems arising from excessive audio dynamics, it is well established in the prior art that automatic devices such as compressors and limiters are frequently used to control the peak and average values of an audio waveform.
Audio compressors fall into two classes which are referred to as feedforward or feedback compressors. Referring to FIGS. 1a and 1b for illustration of the two types, there are shown the basic block diagrams of a feedforward and feedback compressor, respectively. The only difference between a feedforward and feedback compressor is the point where the level detector samples the audio signal. The operation of the compressors of FIGS. 1a and 1b should be obvious enough not to require a detailed description. However, a brief explanation will be offered.
Referring to FIG. 1a which depicts the feedforward compressor, a voltage controlled amplifier (VCA) 2 is connected between an input signal path 1, an output signal path 3, and a gain control signal path 7. The level detector 4 for converting an audio signal into a detected signal suitable to control the gain of the VCA 2 has its input coupled to the input signal path 1 of VCA 2. The output signal of the level detector 4 is coupled to the gain control signal path 7 of the VCA 2. When an audio input signal large enough to exceed the compression threshold established by the level detector 4 appears at the input signal path 1, a detected output signal appears at the VCA gain control signal path 7 reducing the gain of VCA 2 in some proportion to the relative level of the audio input signal. This proportion determines the compression ratio which is expressed as the change of the audio level at the input signal path 1 divided by the resulting change of the audio level at the output signal path 3.
Referring to FIG. 1b which depicts the feedback compressor, one can apply nearly the same explanation as previously given for FIG. 1a. The only difference is that the level detector 4 has its input coupled to the output signal path 3 of VCA 2. In this case, the feedback compressor operates similarly to the feedforward compressor of FIG. 1a except that the effect of gain control feedback creates a servoing mechanism which increases the dynamic stability over that of a feedforward compressor, especially at high compression ratios.
Unfortunately, use of the automatic devices such as the audio compressors can create various unwanted side effects in terms of the sound quality. Deleterious side effects can be severe when the audio peaks, especially transients, are automatically reduced in amplitude by gain reduction. This is because a reduction of gain occurring quickly enough to effectively limit the peak amplitude usually brings about a consequential sudden reduction of the average waveform amplitude for a period of time following the conclusion of the transient peak. This undesired effect is sometimes referred to as "hole punching" since the perception is like a hole punched in the sound level.
In addition, an audio compressor usually has a relatively low compression ratio of about 1.5:1 to about 3:1, and uses a level detector which responds :more to the average value of the sound level. This accomplishes a useful purpose by reducing the average dynamic range of an audio signal while maintaining good sound quality but cannot adequately control the presence of excessively high transient peaks because the compression ratio is too low. In fact, an audio compressor can exaggerate the presence of transient peaks by raising the average level of low level sounds containing transients.
It is therefore desirable to find a new solution for managing the peaks of an audio signal, especially the transients, which can be applied to both feedforward and feedback compressor types.